1. Field of the Invention
The present invention relates to a method of fixing a color screening structure for a color picture tube.
2. Description of the Prior Art
A color picture tube comprises, as shown in FIGS. 9 and 10, a tube body 1 having a faceplate 1P provided over the inner surface thereof with a color fluorescent screen 3, electron guns 4, and a color screening structure 5, such as an aperture grill or a shadow mask, mounted just back of the color fluorescent screen 3 to ensure that each of the electron beans produced by the electron guns 4 strikes only its intended color phosphor dots in the color fluorescent screen 3. The color screening structure 5 comprises a frame 6 consisting of a pair of parallel frame members 6a and 6b extending opposite to each other,and side frame members 6c and 6d extending in parallel to each other so as to interconnect the frame members 6a and 6b , and a grid member 7 provided with a plurality of slits and extended across the frame members 6a and 6b on the side of the frame 6 facing the color fluorescent screen 3. Supporting spring plates 7A, 7B and 7C provided respectively with through holes 8 are welded respectively to the frame member 6a and the side frame members 6c and 6d of the frame 6. The color screening structure 5 is mounted on the tube body 1 with the through holes 8 of the supporting spring plates 7A, 7B and 7C receiving pins 9A, 9B and 9C, respectively, attached to the inner surface of the skirt of the faceplate 1P.
A method of fixing the supporting spring plates 7A, 7B and 7C respectively to the pins 9A, 9B and 9C formed of a metal through laser welding has been proposed to prevent the color screening structure 5 from being dislocated when shocks are applied to the color picture tube, namely, to improve the shock resistance of the color screening structure 5. However, this proposed method has a serious problem that the supporting spring plates 7A, 7B and 7C and the pins 9A, 9B and 9C are melted partly during laser-welding process and contract when cooled after welding, so that the supporting spring plates 7A, 7B and 7C are dislocated relative to the pins 9A, 9B and 9C to dislocate the color screening structure 5 relative to the faceplate 1P.
On the other hand, the color fluorescent screen 3 of the color picture tube is formed by optical means while the faceplate 1P and the color screening structure 5 are held in a fixed positional relation. Therefore, although the color picture tube having the color screening structure attached to the tube body by welding the supporting spring plate 7A, 7B and 7C to the pins 9A, 9B and 9C has excellent shock resistance, it is possible that the color screening structure is misaligned with the faceplate 1P, hence, with the color fluorescent screen 3, at the completion of manufacture and hence electron beams strike wrong color phosphor dots to cause so-called chromatic misalignment. Such a color picture tube is unacceptable.
Japanese Utility Model Publication No. 60-15241 discloses a method of fixing the supporting spring plates to the metallic pins. In this known method, a supporting spring plate 7 is provided with a hexagonal through hole 8 formed in a shape to receive a metallic pin 9 therethrough so that the metallic pin 9 is in contact with every other side of the hexagonal through hole 9 and the supporting spring plate 7 and the metallic pin 9 are welded together by spot welding at the sides q1, q2 and q3 of the hexagonal through hole 8 which are not in contact with the metallic pin 9 as shown in FIG. 11. This known method, however, has practical problems that the gaps between the sides q1, q2 and q3 and the metallic pin 9 must be very small, for example, 0.3 mm or less, to enable satisfactory spot welding, that, in some cases, the sides q1, q2 and/or q3 of the hexagonal through hole 8 which must be separated from the metallic pin 9 is in contact with the metallic pin 9 and that misalignment of the color screening structure can occur due to heating the metallic pin 9 and the supporting spring plate 7 in welding causing the dislocation of the supporting spring plate 7 relative to the metallic pin 9. These problems have become more and ore serious with the progressive improvement of the precision of the color picture tube.